Current transformer

ABSTRACT

In a current transformer of the erect type wherein the primary conductor provided with a predetermined main insulator is extended from one of the primary terminals disposed in the upper oil tank, is passed through the interior of a porcelain tube, is interlinked with the secondary side of the current transformer as is arranged in the lower oil tank, and is returned to the other primary terminal disposed in the upper oil tank, and wherein an insulating liquid is filled in the current transformer, a current transformer wherein the primary conductor is hollow and is formed of two unit hollow conductors substantially different in the amount of generated heat per unit length, and wherein the insulating liquid is circulated in the hollow primary conductor.

Patex ated Aug. 22, 172

4 Sheets-Sheet 1 a v w 4 w a a a r v r v v a A a ll INVENTOR S MmaRu OZAWAIADASHI manna,

MASH-MRO ISHIGAM\ aMmufl; 6 #1101 ATTORNEYS Patented Aug. 22, 1972 3,686,59-

4 Sheets-Sheet 2 j mumfi g INVENTORS MmORU OZAWA'TADASHI Momro,

MASAHIRO ISHIGAMI ("M Q anwww 4, HM

ATTORNEYS;

4 Sheets-Sheet 4.

Patented Aug. 22, 1972 INVENTORS MINORU bzAwA mDAsm MOR IT NASAHIRO ISHIGAMI BY CfQi QAtOKOQQL Q HLQQ ATTORNEYS CURRENT TRANSFORMER BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to current transformer for use in power transmission and distribution circuits, and more particularly to a current transformer for a superhigh-tension and high-current circuit.

2. Description of the Prior Art The recent advancement of the superhigh-tension power transmission technology is remarkable, and a transmission system of 500 kV class is also in practical use. Such transmission systems are at a superhigh-tension, and simultaneously are often at a current as high as above 1,000 a. The current transformer employed as an element of protection for electrical equipment and appliances, such as transformers, used in the system has also become remarkably large in size and heavy in weight.

In general, in thecu rrent transformer used in a superhigh-tension and high-current circuit, the amount of heat generated by the electric resistance of the primary conductor is very large. Accordingly, in the erect type of current transformer wherein the primary conductor is guided from the primary terminal of the upper oil tank through a porcelain tube into the lower oil tank, is

interlinked with the current-transformer secondary is applied with a main insulator conforming to a circuit voltage, so that the thermal radiation is poor, the cooling is made difficult, and that the main insulator suffers from thermal deterioration. For these reasons, an insulating liquid such as mineral oil is generally used, and the whole current transformer becomes as large as 10 m to obvious disadvantages.

In order to eliminate the above-mentioned disadvantages of the erect type of current transformers, there has been suggested the inverted type of current transformer which is constructed such that, within the upper oil tank mounted on the upper end of the porcelain tube being erected on the lower oil tank, the cur rent-transformer secondary side consisting of the iron core and the secondary winding is accommodated so as to downwardly conduct the secondary lead wires inside the porcelain tube, and that the primary conductor penetrating through the upper oil tank is interlinked with the secondary side of the current transformer.

Since the primary conductor is short in the inverted type of current transformer, the amount of generated heat due to the electric resistance during conduction is much smaller than in the erect type, the cooling may be easily carried out, and there is no thermal deterioration of an insulting material such as paper. Since, however, the current-transformer secondary side being a heavy thing is placed on the upper part of the tall porcelain tube reaching 8 m in case of one for the 500 kV class, the porcelain tube becomes easily broken by an earthquake, etc. The mechanical strength is therefore required to be sufficiently reinforced, and hence, the diameter of the porcelain tube should be made large. Disadvantageously, however, this leads to difficulties in manufacture and a comparatively high cost. Another measure in which, in order to increase the earthquakeproof strength of the long porcelain tube, the upper oil tank and an installing mount are coupled using support posts of an insulator, not only renders the current transformer remarkably disadvantageous from the economical viewpoint, but also makes the installation area of the current transformer large.

For this reason, the erect type of current transformer is re-evaluated which may greatly reduce the installation area in comparison with the inverted type and which may be economically manufactured. It is desired that the erect type is so improved as to allow a good cooling of the primary conductor and to allow an easy manufacture. In order to make good the cooling of the erect type of current transformer, various measures have heretofore been suggested.

For example, in a construction, a hollow conductor is usedfor the primary conductor of the erect type of current transformer, an insulating tube for an insulating liquid cooled by cooling means being provided at a lower part of the current transformer or at another ble pipe in order to connect it with plate-like conductors respectively interlinked with the primary terminals and the secondary side of the current transformer, the inner and outer pipes are caused to communicate by pores, and an insulating liquid is circulated by the thermosyphon action of the double pipe.

Although all the above-mentioned current transformers are of the erect type, they should be formed into special structures, which are not suited to the superhigh-tension and high-current use, in order to cool the primary conductor. lnconveniently, the manufacture of the structures is not easy, and yet, requires specific auxiliary equipment and appliances.

As referred to above, in current transformers, especially the erect type of current transformer for a high current of superhigh-tension, the primary conductor and the current-transformer secondary side at an electric potential close to the earth one are insulated by the main insulator formed of a plurality of layers of shield, insulating paper, etc. Herein, there is a fear that the main insulator and other insulating materials thermally deteriorate due to the generated heat of the primary conductor, leading to the trouble of dielectric breakdown. To fully eliminate the fear and to maintain the function, it is strongly desired to make it possible to cool the primary conductor without any complicated and large construction.

SUMMARY OF THE INVENTION An object of the present invention is to make it possible to easily manufacture a current transformer of the erect type, especially for the superhigh-tension and high-current use, having a construction wherein a hollow primary conductor provided with a predetermined main insulator is guided from one of the primary terminals disposed in the upper oil tank, is passed through the interior of a porcelain tube, is interlinked with the current-transformer secondary side arranged in the lower oil tank, so as to be inductively coupled therewith, and is returned to the other primary terminal of the upper oil tank, and wherein a predetermined amount of insulating liquid is filled in the lower oil tank, the porcelain tube and the upper oil tank.

Another object of the present invention is to provide a current transformer of the erect type which may be manufactured in a smaller size and more economically than in prior-art current transformers.

. Still another object of the present invention is to provide a current transformer of the erect type wherein the hollow primary conductor is formed of at least two unit hollow conductors being substantially different in the amount of generated heat per unit length, whereby an insulating liquid is efliciently circulated in the hollow primary conductor to effect a good cooling, dielectric breakdown due to the thermal deterioration of the main insulator, etc., is not caused, and the expected function may be maintained.

The above-mentioned and other objects and features of the present invention will be apparent from the following description taken in conjunction with the respective embodiments.

A characterizing feature of the present invention resides in that the current transformer comprises a lower oil tank in which at least one secondary side member of the current transfonner is arranged, a porcelain tube erected at the upper part of the lower oil tank, an upper oil tank fixed to the upper end of the porcelain tube and provided with at least two primary terminals, a predetermined amount of insulating liquid filled into the lower oil tank, the porcelain tube and the upper oil tank, and a hollow primary conductor extended from one of the primary terminals, disposed in the upper oil tank, through the interior of the porcelain tube to the lower oil tank, interlinked with the secondary side of the current transformer at least once, and returned to the other primary terminal of the upper oil tank, and that the hollow primary conductor is formed of at least two unit hollow conductors substantially different in the amount of generated heat per unit length, the insulating liquid being adapted to be naturally circulated inside the hollow primary conductor by a temperature difference.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a longitudinal section of a current transformer of the present invention.

FIG. 2 is a front view of a hollow primary conductor used in the current transformer of the present invention, and illustrating that the hollow primary conductor is formed of at least two unit hollow conductors differing in the amount of generated heat per unit length.

FIGS. 3, 4 and 5 are cross sections of respectively different, hollow primary conductors in FIG. 2 as viewed from the line A A, in which FIG. 3 illustrates a hollow primary conductor formed of right and left unit hollow conductors different in thickness, FIG. 4 illustrates a dual hollow primary conductor consisting of respectively two right and left unit hollow conductors, and FIG. 5 illustrates a hollow primary conductor whose right and left unit hollow conductors are made at an identical thickness and of different metal materials.

FIGS. 6 and 7 are longitudinal-sections each showing another embodiment of the current transformer of the present invention.

FIGS. 8 and 9 are front views showing respectively different, lowerparts of the current transformer of the present invention in detail partially in longitudinal section.

FIG. 10 is a front view showing an eyebolt-shaped primary conductor applicable to the current transformer of the present invention.

FIGS. 11 and 12 are cross sections of respectively different, eyebolt-shaped primary conductors in FIG. 10 as viewed from the line B B, in which FIG. 11 illustrates an eyebolt-shaped primary conductor whose extension part circular in section consists of two units, while FIG. 12 illustrates such a part consisting of four units.

DESCRIPTION OF THE PREFERRED EMBODIIWENTS FIG. 1 shows a current transformer 1 of the erect type according to the present invention. As is well known, an erect type is constructed as below.

In a lower oil tank 2, the secondary side 6 of the current transformer is arranged which is formed of an iron core 7 and a secondary winding 8 and which is led by means of lead wires to a terminal box (not shown) disposed in the lower oil tank 2. A porcelain tube 3 is erected on the lower oil tank 2. Further, an upper oil tank 4 provided with at least two primary terminals 11 is fixed onto the upper end of the porcelain tube 3. The interior of the lower oil tank 2, the porcelain tube 3 and the upper oil tank 4 is filled with an insulating liquid 10 such as mineral oil, with a space of small volume 16 left.

A hairpin-shaped primary conductor 5 connected to the respective primary terminals 11 of the upper oil tank 4 by connecting pieces 12 is formed into a hollow structure in accordance with the present invention, and will be hereinafter stated in detail.

The hollow primary conductor 5 connected to one of the primary terminals 11 is conducted through the interior of the porcelain tube 3 to the lower oil tank 2, it is interlinked at least once with the secondary side 6 of the current transformer as disposed within the lower oil tank 2, so as to be inductively coupled therewith, and it is returned to the other primary terminal 11. Since the primary conductor 5 is at a superhigh voltage, a main insulator 9 formed of insulating paper or a plurality of conductive layers is applied thereon for the insulation when it is conducted downwards and is interlinked with the secondary side 6 at an electric potential close to the earth potential. The main insulator 9 has heretofore been well known, and hence, it is not especially described in detail.

The current-transformer secondary side 6 in the lower oil tank 2 and the primary conductor 5 interlinked therewith and provided with the main insulator are mechanically supported by support insulators l4 and of, for example, insulating timber. As is more clearly illustrated in FIG. 2, the hollow primary conductor 5 is formed of at least two unit hollow conductors 5A and 5B which substantially differ in the amount of generated heat per unit length as is based on the electric resistance at conduction. Utilizing the difference in the amount of generated heat between both the unit hollow conductors, the insulating liquid 10 is circulated. Means is therefore executed, for example, such that the amount of generated heat per unit length of one unit hollow conductor 58 is larger than that of the other unit hollow conductor 5A. The dividing point 5C or 5C between the respective unit hollow conductors 5A and 58 forming the hollow primary conductor 5 is suitably determined so as to be located inside or outside the main insulator 9, in consideration of various conditions such as insulating operations for the main insulator'9 and temperature difl'erences appearing-in the primary conductor 5. j

The at least two unit hollow conductors 5A and 58 forming the hollow primary cohductorS have executed the means to: render the amounts of generated heat per unit length substantially different,.such as employment of different configurations or dimensions in section or of diflerent materials for the unit hollow conductors. More concretely, as shown in FIG. 3, the respective unit hollow conductors 5A and 58 approximately equal in the outside diameter have their thicknesses t and made different. If, for example, t t a current density in the left unit hollow conductor 5A becomes low and that in the right unit hollow conductor 5B becomes high since the conduction current of the hollow primary conductor 5 is the same at the respective parts. As a result, the amount of generated heat per unit length becomes much larger in the unit hollow conductor 53 than in the unit hollow conductor 5A.

In an example which the inventors carried out, temperature differences of 2 to 3 C. were produced with a hollow primary conductor 5 in which the outside diameters of unit hollow conductors 5A and 5B respectively made of copper were approximately 5 inches, the thickness t, was approximately one-half inch, the thickness t was approximately one-third inch, and the unit hollow conductors were made integral at the lower divisional point 5C by welding.

The section of the hollow primary conductor as illustrated in FIG. 4 is applied to a'current transformer of higher conduction currents than in the hollow primary conductor in FIG. 3. In this case, respectively two unit hollow conductors 5A and 5B are used, the thickness of the right conductors and that of the left conductors are made different as described above, and each pair of unit conductors are maintained at the same potential with a copper plate 5D or the like. This construction may be diverted for a current transformer of double ratio through a simple modification. In case where the hollow primary conductor 5 is formed using unit hollow conductors 5A and 5B which have equal outside diameters and which are made of different types of metal, e.g., copper and a copper alloy, the thicknesses t and t, of the respective unit hollow conductors may be made equal as shown in FIG. 4. The unit members are made integral by welding or other joining means. Thus, the amounts of generated heat per unit length of both the unit hollow conductors 5A and 5B may also be made different on account of the difference in electric resistance between the materials used.

The hollow primary conductor 5 applied to the current transformer l of the present invention may be prepared such that a single copper tube is employed, it is subjected to such machining as drawing and a contraction of the area thereby changing the configuration or dimensions of the section, and that it is thus made divisible into the respective unit hollow conductors 5A and 58 at the dividing point 5C at a suitable position. Alternatively, at least two unit hollow conductors 5A and 58 having previously been separately formed of the same sort or different sorts of materials are joined at a suitable dividing point 5C, e.g., in the vicinity of the lower part interlinked with the secondary side 6 of the current transformer or at that part of the primary conductor at which it is substantially rectilinear, by the use of suitable auxiliary joining means and by welding, hard soldering or any other suitable method, and thus, they are formed into an integral body. Between the two measures, no difference occurs in accomplishing the present invention.

Since, in the current transformer 1 of the present invention, the hollow primary conductor 5 is formed as described above or such hollow primary conductors 5 are suitably combined, a substantial difference appears between the amounts of generated heat per unit length of the left and right unit hollow conductors 5A and 58. One unit hollow conductor 5A, for example, is therefore apparently larger in the amount of heat generation than the other unit hollow conductor 58 during conduction, so that the insulating liquid 10, such as mineral oil, being present in the hollow portions is naturally circulated so as to flow upwards inside one of the unit hollow conductors and downwards inside the other as shown by arrows in FIG. I, because of a difference in the lowering of density due-to heating. The cooling action-of eflectively removing the generated heat of the primary conductor 5 is therefore carried out. Accordingly, even if the construction of the current transformer 1 is not especially modified, the temperature rise of the primary conductor 5 is suppressed within a suitable range, and the thermal deterioration of the main insulator 9 is prevented. Furthermore, not only the effect of preventing dielectric breakdown due to the thermal deterioration is enhanced, but also the manufacture of the current transformer 1 becomes extremely easy.

The greater part of the amount of generated heat of the primary conductor 5 is radiated into the atmospheric air from the upper oil tank 4, and the temperature rise is thus suppressed. The cooling efiect may therefore be more enhanced by providing cooling means, such as a radiating piece 13 of suitable shape, in the upper oil tank 4. In addition, in order to circulate the insulating liquid 10 of comparatively low temperature within the primary conductor 5 and to thereby attain the enhancement of the cooling efiect, there may be equipped guide means such as a guide plate 17 for the insulating liquid 10 as is supported by the primary conductor 5 or the upper oil tank 4, and openings or cuts provided at the upper end of the unit hollow conductor A.

In order to perform the cooling of the hollow primary conductor 5 owing to an especially effective circulation of the insulating liquid 10, it is desired that the amount of generated heat per unit length of one conductor unit, e.g., 5B, is made larger by at least percent than that of the other conductor unit 5A, to raise the circulating action of the insulating liquid 10 flowing inside the 'conductor units. However, even when a difierence in the amount of generated heat appears more than is required, the amount of circulation of the insulating liquid 10 is not remarkably increased, but on the contrary, the total loss of the current transformer 1 is consequently increased. Therefore, the amounts of generated heat are suitably selected in sufficient consideration of both the cooling effect and the total loss.

In order to make the circulation of the insulating liquid 1-0 effective to improve the cooling of the primary conductor 5, a hollow extension tube 50 is provided, as shown in FIG. 6, at the upper end of one conductor unitSB larger in the amount of generated heat per unit 7 length. A suitable difference in height is thus caused between the respective conductor units, whereby the circulating action of the insulating liquid flowing inside the hollow primary conductor 5 is improved by the socalled chimney effect thus enhancing the cooling efficiency. According to a further measure, in addition to the guide means 17 as previously stated, a radiating fin or the like is provided in the vicinity of the upper end of the conductor unit 5A smaller in the amount of generated heat per unit length than the other. The thermal radiationis thus increased, so that the density of the insulating liquid inside the conductor unit 5A may be more diflerent from that inside the other conductor unit 53 thereby promoting the downward movement of the insulating liquid inside the former conductor unit. This measure improves the cooling action in combination with the construction as described above. In a still further measure, as shown in FIG. 7, an insulating liquid of low boiling point, such as freon, is employed for the insulating liquid 10, while the hollow extension tube 50 as in FIG. 6 is projected above the liquid level. Thus, the gasification and liquefaction of the insulating liquid 10 are repeated. This construction is also efiective in improving the cooling action.

Where, as in the current transformers 1 illustrated in FIGS. 6 and 7, the hollow extension tube 50 is provided in one unit hollow conductor 58 constituting the hollow primary conductor 5 and being larger in the amount of generated heat per unit length than the other and a difference in height is thereby caused between both the unit hollow conductors, the conductor unit 5B may be merely extended with a connecting piece 12 connected as illustrated by broken lines. Alternatively, there may be attached the hollow extension tube 50 which is separately formed of a suitable material such as a thermally insulating one.

When a plurality of secondary side members 6 of the current transformer as are interlinked with the foregoing hollow primary conductor 5 at least once, are used in order to attain improvements in the characteristics of the current transformer, they are effectively arranged as illustrated in FIGS. 8 and 9 by way of examples. Thus, the manufacture is facilitated, while the amount of use of the insulating liquid 10 is reduced.

More specifically, there are formed in combination the first oil tank unit '20 in which the lower oil tank 2 is formed so as to be capable of horizontally arranging at least two secondary side members 6 of the current transformer, and the second oil tank unit 21 which is supported on a mount 22 and which is substantially fitted with the primary conductor 5 interlinked with the secondary side members of the current transformer. In this case, it is also possible to allow the secondary side members 6 of the current transformer to be arranged in the second oil tank unit 21.

The hollow primary conductor 5 led into the lower oil tank 2 is supported by an insulator 23, such as a press board plate, which is arranged in the second oil tank unit 21. On the other hand, the secondary side members 6 of the current transformer horizontally arranged in the first oil tank unit 20 are supported by an insulator 24 arranged at a ring-shaped part of the primary conductor 5 and an'insulator 25 arranged at a junction part of the respective oil tank units.

.An eyebolt-shaped primary conductor 30 shown in FIG. 10 may also be used for the current transformer l of the present invention.

The eyebolt-shaped primary conductor 30 is formed such that semi-circular extension portions 31A and 31B respectively connected to the primary terminals and a hollow interlinkage portion 32 interlinked with the secondary side of the current transformer at least once and inductively coupled therewith are joined by, e.g., welding. It has a predetermined main insulator 9 applied thereon, and is used. The right and left extensions 31B and 31A forming part of the primary conductor 30 are electrically insulated by an insulating piece 34, and therewith, means to render the amounts of generated heat per unit length substantially different is applied according to the present invention.

More specifically, as shown in FIGS. 10 and 11, a plate-shaped conductor 33 is joined only to one semicircular extension 31B by welding or the like. Thus, the current density of the extension 31B is made lower than that of the other extension 31A, the amount of generated heat per unit length as appears in the former extension during conduction is made smaller than in the latter extension, and the insulating liquid is circulated from the extension 318 through the interlinked portion 32 to the otherextension 31A. The cooling is thereby efl'ected efficiently.

With the eyebolt-shaped primary conductor 30 as described above, when it is intended to be interlinked twice with the secondary. side of the current transformer (for example, in the case of the double ratio) in order to enhance electric characteristics, the primary conductor is formed using extension portions 31A, 31A and 31B, 318 which, as shown in FIG. 12, are respectively divided into two parts on each of the right and left sides, with the insulating piece 34 interposed therebetween, and the interlinked portion (not shown) divided into two parts with the insulating piece interposed therebetween.

In case where the eyebolt-shaped primary conductor as stated above is used in the current transformer, the section of the extended part is circular, and hence, a larger one may be accomodated in the same porcelain tube. Therefore, an increase in'the current capacity and the manufacture of a double-ratio construcu'on are easily attained. Moreover, the cooling may be efficiently carried out by the circulation of the insulating 6. A current transformer comprising: liquid. It is a matter of course that, herein, the difa. a lower oil tank in which the secondary side of said ference in the amount of generated heat between the current transformer is arranged, right and left conductors 31A and 31B may be proa. said lower oil tank consisting of a first oil tank unit vided by different thicknesses, etc.,- as previously which is formed so as to be capable of horizontally stated. an in at least two secon side m m rs of w claim; d u ent transformer, an d h secon ofi tank A cum-em transformer comprising: unit which is formed so as to be substantially fitted a. a lower oil tank in which at least one secondary wlthapnmary conductor menlwned side member of said current transformer is ara Porcelam, tube erected lower f ranged c. an upper oil tank fixed at an upper end of said porb. a porcelain tube erected on said lower oil tank, S g and provded at least two pnmary c. an upper oil tank fixed at an upper end of said porcelajn tube and provided with afleast two primary d. an insulating liquid filled in said lower oil tank, tenninals, said porcelain tube and said upper 011 tank, and an insulating liquid n in Said lower oil tank e. at least one hollow primary conductor provided Said porcelain tube and Said upper oil tank, and with a predetermined main insulator, WhlCh cone. at least one hollow primary conductor provided F from of smd pnmary f f with a predetermined main insulation, which con- P' upper 011 throng}? the F E ductor extends from one of said primary terminals of f P 'Q Q tube t0 f 1 Wr 011 tank, IS inprovided in said upper oil tank through the interior terlmked Yvlth Said respecuve secondary of said porcelain tube to said lower oil tank, is inbers of sad current "aPsmrmer least i i terlinked with said secondary side member of said rePums to h other Pnmary termmal pmv'ded In current transformer at least once, and returns to upper 011m, the other primary terminal provided in a upper e said hollow primary conductor including at least oil tank two unit hollow conductors jOlI'lCd together which c. said hollow primary conductor being formed of at are respecfively formed at q thlckPesses least two unit hollow conductors which are thereby being made substantially d fierent in the stantially different in the amount of generated heat amount of generated heat f a i per unitlength, and being adapted to circulate said being adapted to circulate said insulating liquid insulating liquid therein. therem- A Currant transformer according to claim 1 7. A current transformer according to claim 6,

h i id hollow primal-y conductor comprises at wherein said hollow primary conductor is provided least two unit hollow conductors joined together which Y a hollow extenslo tube at "PPS end of are respectivelyformed at different thicknesses thereby hollow conductor larger Sal amount 0 being made substantially different in the amount of generated heat P nitlength' generated heatpelunitlength 8. A current transformer according to claim 6,

3. A current transformer according to claim 1, b said hollow q q is Provided wherein said hollow primary conductor comprises at gufde means for Sam msulatmg l at an end of least two unit hollow conductors joined together which 40 Sand umt hollow conqucwr smaller Sald amount of are respectively formed of metal materials of different generated h P g electric resistances thereby being made substantially cufrent transfcfrmer accgrding i :5 different in the amount of generated heat per unit whefem P con actor J length portion of said unit hollow conductors posltioned 1n- 4 A current transformer according to claim 1 side said main insulator, said unit hollow conductors wherein said hollow primary conductor comprises at '9 dlfferem amount of generated heat P least two unit hollow conductors which are respectively lengthformed at different sectional configurations thereby cfment transttomer according to im being made substantially different in the amount of wherein said hollow primary conductor has a oined generated heat per unit length portion of said umt hollow conductors positioned out- 5 A cum-em u-ansfbmer according to claim 1 side said main insulator, said unit hollow conductors wherein said hollow primary conductor is made an being different in said amount of generated heat P eyebolt shape which consists of an interlinkage portion lengthinterlinked with said secondary side member of said 9 transformer accordmg 9 clalm 6, current transformer at least once and extension porwherem {PP Q Provlded with coolmg tions respectively connected with said primary termeans for 531d msulatm? q di 1 6 minals, the amounts of generated heat-per unit length Q 'f f 9 9 to c f said respective extension portions being made Sub wherein said insulating liquid 15 of low boiling point. stantially different. 

1. A current transformer comprising: a. a lower oil tank in which at least one secondary side member of said current transformer is arranged, b. a porcelain tube erected on said lower oil tank, c. an upper oil tank fixed at an upper end of said porcelain tube and provided with at least two primary terminals, d. an insulating liquid filled in said lower oil tank, said porcelain tube and said upper oil tank, and e. at least one hollow primary conductor provided with a predetermined main insulation, which conductor extends from one of said primary terminals provided in said upper oil tank through the interior of said porcelain tube to said lower oil tank, is interlinked with said secondary side member of said current transformer at least once, and returns to the other primary terminal provided in said upper oil tank, e''. said hollow primary conductor being formed of at least two unit hollow conductors which are substantially different in the amount of generated heat per unit length, and being adapted to circulate said insulating liquid therein.
 2. A current transformer according to claim 1, wherein said hollow primary conductor comprises at least two unit hollow conductors joined together which are respectively formed at different thicknesses thereby being made substantially different in the amount of generated heat per unit length.
 3. A current transformer according to claim 1, wherein said hollow primary conductor comprises at least two unit hollow conductors joined together which are respectively formed of metal materials of different electric resistances thereby being made substantially different in the amount of generated heat per unit length.
 4. A current transformer according to claim 1, wherein said hollow primary conductor comprises at least two unit hollow conductors which are respectively formed at different sectional configurations thereby being made substantially different in the amount of generated heat per unit length.
 5. A current transformer according to claim 1, wherein said hollow primary conductor is made an eyebolt shape which consists of an interlinkage portion interlinked with said secondary side member of said current transformer at least once and extension portions respectively connected with said primary terminals, the amounts of generated heat per unit length of said respective extension portions being made substantially different.
 6. A current transformer comprising: a. a lower oil tank in which the secondary side of said current transformer is arranged, a''. said lower oil tank consisting of a first oil tank unit which is formed so as to be capable of horizontally arranging at least two secondary side members of said current transformer, and a second oil tank unit which is formed so as to be substantially fitted with a primary conductor mentioned below, b. a porcelain tube erected on said lower oil tank, c. an upper oil tank fixed at an upper end of said porcelain tube and provided with at least two primary terminals, d. an insulating liquid filled in said lower oil tank, said porcelain tube and said upper oil tank, and e. at least one hollow primary conductor provided with a predetermined main insulator, which conductor extends from one of said primary terminals provided in said upper oil tank through the interior of said porcelain tube to said lower oil tank, is interlinked with said respeCtive secondary side members of said current transformer at least once, and returns to the other primary terminal provided in said upper oil tank, e''. said hollow primary conductor including at least two unit hollow conductors joined together which are respectively formed at different thicknesses thereby being made substantially different in the amount of generated heat per unit length, and being adapted to circulate said insulating liquid therein.
 7. A current transformer according to claim 6, wherein said hollow primary conductor is provided with a hollow extension tube at an upper end of said unit hollow conductor larger in said amount of generated heat per unit length.
 8. A current transformer according to claim 6, wherein said hollow primary conductor is provided with guide means for said insulating liquid at an end of said unit hollow conductor smaller in said amount of generated heat per unit length.
 9. A current transformer according to claim 6, wherein said hollow primary conductor has a joined portion of said unit hollow conductors positioned inside said main insulator, said unit hollow conductors being different in said amount of generated heat per unit length.
 10. A current transformer according to claim 6, wherein said hollow primary conductor has a joined portion of said unit hollow conductors positioned outside said main insulator, said unit hollow conductors being different in said amount of generated heat per unit length.
 11. A current transformer according to claim 6, wherein said upper oil tank is provided with cooling means for said insulating liquid.
 12. A current transformer according to claim 6, wherein said insulating liquid is of low boiling point. 